1. Field of the Invention
This invention relates to silicone elastomer stock which has improved extrusion characteristics.
2. Background Information
One of the uses of silicone elastomer stock is extrusion into elastomeric shapes. The ability to be extruded into the desired shape without excessive die swell and with smooth faces and edges is a desirable characteristic. Little has been taught concerning means for improving extrudability of silicone rubber stocks other than methods of improving the handling of silicone stocks.
U.S. Pat. No. 2,721,857, issued Oct. 25, 1955, teaches organopolysiloxane rubber stocks having improved handling characteristics. These stocks consist essentially of an organopolysiloxane having from 1.99 to 2.0 inclusive radicals on silicon, 0.005 to 0.09 part by weight of a boron compound, filler, and vulcanizing agent.
U.S. Pat. No. 3,070,560, issued Dec. 25, 1962, teaches improved handling stocks comprising polydiorganosiloxane endblocked with either R.sub.3 Si- or R'O-groups, treated silica filler, hydroxylated siloxane, and boron compound.
U.S. Pat. No. 3,734,882, issued May 22, 1973, teaches improved handling properties and improved processability by adding organic titanate to the composition of polydiorganosiloxane gum and reinforcing filler.
U.S. Pat. No. 4,010,136, issued Mar. 1, 1977, to Blizzard et al. discloses that when a small amount of polytetrafluoroethylene powder is mixed with certain curable polydiorganosiloxane compositions and the resulting mixture is extruded, the resultant article has a greatly reduced amount of porosity and a smoother surface.
U.S. Pat. No. 4,361,655, issued Nov. 30, 1982, to Tyler discloses that the addition of from 1 to 10 percent of particulate vulcanized silicone rubber added to unvulcanized silicone rubber gives improved processing; i.e., faster rates.
None of these references is directed to a method of improving the extrudability of silicone stocks as measured by their ability to make smooth surfaced extrusions in the shape of acute angles as found in a Garvey die.
After the discoveries leading to this invention were known, other prior art became of interest.
U.S. Pat. No. 4,122,247, issued Oct. 24, 1978, to Evans teaches that it is preferred that there be less than 10 parts per million of water present in cyclic siloxanes used to make polymers. He states that if there is substantially more than this amount of water present in the cyclic siloxanes, then the desired low molecular weight oil or high molecular weight gum will not be formed in commercially attractive yields. Removal of all but traces of water is accomplished by heating to 110.degree. C. or above with a nitrogen purge.
U.S. Pat. No. 4,551,515, issued Nov. 5, 1985, to Herberg et al. teaches that during the polymerization of polydiorganosiloxanes, the mixture of methyl tetramer and chain stopping agent is preferably passed through a drying means. The reason that it is desirable to pass the reactants through drying means to eliminate water which will terminate or chainstop diorganopolysiloxane polymer with silanol groups and thus reduce the viscosity of the polymer and change the nature of its interaction with a filler.